Method and otdr apparatus for optical cable defect location with reduced memory requirement

ABSTRACT

Optical time domain reflectometer (OTDR) systems, methods and integrated circuits are presented for locating defects in an optical cable or other optical cable, in which a first optical signal is transmitted to the cable and reflections are sampled over a first time range at a first sample rate to identify one or more suspected defect locations, and a second optical signal is transmitted and corresponding reflections are sampled over a second smaller time range at a higher second sample rate to identify at least one defect location of the optical cable for relaxed memory requirements in the OTDR system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/515,724, filed Oct. 16, 2014, the entirety of which is incorporatedherein by reference.

BACKGROUND

Optical time domain reflectometers or OTDRs are used to locate defectsin an optical cable by transmitting pulses into the cable and receivinglight pulses due to Rayleigh scattering, reflection or other effects.The received light signal is analyzed by correlating time delays betweenthe input signal and the reflected light signals. Certain OTDR systemsinclude displays for presenting loss curves to a user showing tracediscontinuities corresponding to defect locations along the length of anoptical fiber under test. The design specifications of the OTDR systemare related to the speed of light traveling through the cable, theoverall length of the cable, and the desired resolution or accuracy ofthe estimate of the location (distance) of the identified fault(s). Itis desirable to provide an analysis with a relatively small resolutionin order to facilitate assessment of defects and remedial efforts torepair damaged optical cables. For example, it may be desirable toidentify the defect location within a resolution distance of 0.5 m todirect service personnel to the appropriate location for maintenance orrepair. However, long cable lengths in combination with relatively smallresolution distances leads to analysis of a large sample of data, andthe OTDR system must store the sample data in a relatively largeelectronic memory. Large memories, in turn occupy significant areas onintegrated circuits, and consumed power, whereby a need remains forimproved OTDR techniques and apparatus for locating defects in anoptical cable with relaxed memory requirements.

SUMMARY

Presently disclosed embodiments reduce the memory requirements for OTDRsystems through a multi-step search technique including a coarse searchto identify suspected defect locations and one or more fine orhigh-resolution searches to confirm or refine a defect location and/orto identify and locate further defects. OTDR systems, methods andintegrated circuits are provided in which a first optical signal istransmitted to the cable and reflections are sampled over a first timerange at a first sample rate to identify one or more suspected defectlocations. A second optical signal is transmitted and correspondingreflections are sampled over a smaller second time range at a highersecond sample rate to identify one or more defect locations of theoptical cable. The use of low-resolution or “coarse” initial searchreduces the amount of memory required to store sample data, particularlyfor long optical cable lengths, and the targeted “fine” search employshigher sample rates to provide the desired resolution for defectlocation identification. In certain embodiments, different pulse widthscan be employed in the input signal for the first and second searches tofacilitate differentiating between multiple closely spaced defectlocations.

DESCRIPTION OF THE VIEWS OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations in detail, which are indicative of several ways in whichthe various principles of the disclosure may be carried out. Theillustrated examples, however, are not exhaustive of the many possibleembodiments of the disclosure. Other objects, advantages and novelfeatures of the disclosure will be set forth in the following detaileddescription when considered in conjunction with the drawings, in which:

FIG. 1 is a schematic diagram showing an OTDR system with an integratedcircuit implementing a multistep defect location process;

FIG. 2 is a flow diagram showing a process for locating defects in anoptical cable;

FIG. 3 is a graph showing coarse OTDR processing over a first time rangein the system of FIG. 1;

FIG. 4 is a graph showing fine OTDR processing over a smaller secondtime range at a higher sample rate to identify a single defect locationin the system of FIG. 1;

FIG. 5 is a graph showing coarse OTDR processing to identify multiplesuspect defect locations in the system of FIG. 1;

FIG. 6 is a graph showing fine OTDR processing to identify multipledefect locations in the system of FIG. 1;

FIG. 7 is a graph showing use of a first pulse width to identify asingle defect location in the system of FIG. 1;

FIG. 8 is a graph showing use of a first pulse width to attempt toidentify multiple defect locations in the system of FIG. 1; and

FIG. 9 is a graph showing use of a second narrower pulse width todistinctly identify multiple defect locations in the system of FIG. 1.

DETAILED DESCRIPTION

One or more embodiments or implementations are hereinafter described inconjunction with the drawings, wherein like reference numerals are usedto refer to like elements throughout, and wherein the various featuresare not necessarily drawn to scale.

FIG. 1 shows an OTDR system 100 for identifying locations of one or moredefects along the length L of an optical cable 102. The system 100includes a splitter or circulator 104 through which an optical or lightsource 106 provides an optical transmit signal to a first end 102 a ofthe optical cable 100 and from which an optical sensor 108 receivesbackscattered or reflected light from the first end 102 a. An OTDRintegrated circuit (IC) 110 is provided in the embodiment of FIG. 1 withsuitable connections for interfacing with the optical source 106 and theoptical sensor 108, as well as communications connections forinterfacing with a display 160 to provide data and/or graphical curves162 and other information to a user, along with power and groundconnections (not shown) for operating the various components within theIC 110. The OTDR IC 110 also includes a processor 120 providing outputdata to the display 160 as well as various control signals or values122-128 for operating a transmit or transmitter circuit 130 and areceive or receiver circuit 140.

The transmit circuit 130 in the example embodiment operates according toa transmit enable signal 122 (TXEN) from the processor 120 to provide orgenerate a transmit signal 136 defining at least one pulse to operatethe optical source 106 for transmitting optical signals at the first end102 a of the optical cable 102 via the splitter 104. In addition, theexample transmit circuit 130 includes a pulse or burst generator circuit132 with a pulse width control input receiving a pulse width controlsignal or value 124 (PW) from the processor 120 to control a pulse widthparameter of the transmit signal 136 and thus a pulse width of thetransmitted optical signal provided by the optical source 106. Thetransmit circuit 130 further includes a digital to analog converter 134(DAC) providing the transmit signal 136 as a current signal to drive theoptical source 106. Any suitable optical source 106 can be used, such asa laser diode or a P-I-N diode which provides a light output or opticalsignal to the splitter 104 for transmission to the first end 102 a ofthe optical cable 102.

The transmitted optical signal travels at the speed of light along thelength L of the connected optical cable 102 between the first end 102 aand a second end 102 b from left to right in the figure, and is thenreflected from the second end 102 b from left to right back to the firstend 102 a, with reflection or back scattering occurring at one or moredefect locations DL1 along the length L of the cable 102. The transmitcircuit 130 generates any suitable form of single or multi-pulsetransmit signal 136 to provide a single pulse with a controlled pulsewidth or multiple pulses, such as a pseudorandom binary sequence (PRBS)signal of a controlled energy, to create the optical transmit signal atthe output of the optical source 106. The splitter 104 providesisolation between the transmit circuit 130 and the receive circuit 140,with received optical pulses being coupled from the optical cable 102 tothe receive circuit 140 through the splitter 104 to provide an inputoptical signal to the sensor 108. Any suitable optical sensor 108 can beused, such as an avalanche photodiode or P-I-N diode that convertsoptical pulses to current pulses to provide a receive signal 142.

The receive circuit 140 operates to sample the received current outputsignal 142 provided by the sensor 108, with a signal conditioningcircuit 144 including a trans-impedance amplifier (TIA) to convert thecurrent signal 142 to a voltage output representing the received opticalreflections. The signal conditioner 144 may further include a low noiseamplifier (LNA) and an anti-aliasing low pass filter or an analog todigital converter 146 (ADC) with suitable sample and hold front-endcircuitry to sample the filtered signal at a sample rate having acontrolled sample period set according to a control signal or value 128(Ts) from the processor 120. In addition, the receive circuit 140 isselectively enabled or disabled according to a receive enable signal 126(RXEN) from the processor 120. In operation when enabled via the signal126, the receive circuit 140 samples the receive signal 142 at aselectable sample rate, and provides sample data as converted binaryvalues directly to an optional on-board electronic memory 147 and/or toa memory 150 via the processor 120, where the sample data corresponds tothe optical reflections received by the optical sensor 108 and sampledvia the ADC 146.

The electronic memory 147 and/or 150 stores the receive samples, withthe optional on-board memory 147 in certain embodiments performingreal-time accumulation and averaging synchronously with respect to thetransmitted output from the optical source 106. The processor 120 incertain embodiments operates according to program instructions 152stored in the memory 150 in order to implement control and signalprocessing functions for the OTDR IC 110, including a sample ratecontrol component 154, a timing control component 156 and a correlationcomponent 158, which may operate on received samples 159 stored in thememory 150 and/or on received sample data stored in the ADC memory 147.

In one non-limiting example, the processor 120 implements instructionsof the control and signal processing components 152 from the memory 150,including selective provision of control signals and/or values 122 and124 to operate the transmit circuit 130 and signals and/or values 126and 128 to operate the receive circuit 140. In addition, the processor120 implements instructions for analysis of data 148 sampled by thereceive circuit 140, whether stored in the local memory 147 of thereceive circuit 140 and/or receive sample data 159 stored in the mainmemory 150. In one embodiment, the processor 120 analyzes sample datastored in the memory 147, which is integrated into the ADC converter 146in order to temporally correlate a set of sample data with an opticalsignal transmitted under control of the processor 120 by the transmitcircuit 130 and the optical source 106. In certain implementations,moreover, the dynamic range of the receiver circuit 140 may be improvedby averaging signal data 142 from the optical sensor 108, with thetransmit circuit 130 transmitting a sequence of optical signals to theoptical cable 102, and the output of the sensor 108 being sampled andconverted by the ADC 146. In this case, the memory 147 synchronouslyaccumulates and averages sample values for subsequent correlation by theprocessor 120. Such accumulated sample data, moreover, can besubsequently transferred to the memory 150 for processing by theprocessor 120. In other possible embodiments, the sample data isprovided by the ADC 146 to the processor 120, whether averaged byintermediate accumulation in the local memory 147 or not, and the sampledata is stored as receive samples 159 in the memory 150, with theprocessor 120 performing the correlation analysis based on the data 159stored in the memory 150.

Referring also to FIGS. 2-4, FIG. 2 illustrates a process or method 200for locating defects in the optical cable 102, which can be implementedin certain embodiments using the OTDR system 100 of FIG. 1. FIG. 3provides graphs 300, 310 and 320 showing coarse OTDR processing over afirst time range R1 using a relatively low sample rate or samplefrequency Fs=1/TS1 in the system 100, and FIG. 4 provides graphs 400,410 and 420 showing fine OTDR processing over a smaller second timerange R2 at a higher sample rate (Fs=1/TS2) to identify a single defectlocation (location DL1 in FIG. 1). While the method 200 is illustratedin FIG. 2 and described in the form of a series of acts or events, thevarious methods of the present disclosure are not limited by theillustrated ordering of such acts or events except as specifically setforth herein. In this regard, except as specifically providedhereinafter, some acts or events may occur in different order and/orconcurrently with other acts or events apart from those illustrated anddescribed herein, and not all illustrated steps may be required toimplement a process or method in accordance with the present disclosure.The illustrated methods may be implemented in hardware as illustratedand described above, and/or using processor-executed software,processor-executed firmware, FPGAs, logic circuitry, etc. orcombinations thereof, in order to provide the OTDR functionalitydescribed herein for locating defects in an optical cable 102, althoughthe present disclosure is not limited to the specifically illustrated ordescribed applications and systems.

In operation according to one embodiment, the processor 120 implements acoarse or first identification step 210 in FIG. 2, followed by asubsequent fine identification step 230. The initial identification at210 includes setting a receive sample period TS1 at 211 for operation ofthe ADC 146, for example, by the processor 120 providing a signal orvalue 128 to the receive circuit 140 indicating the selected initialsample period TS1 (or its reciprocal as a sample frequency Fs1=1/TS1).In certain embodiments, moreover, the processor 120 may optionally set apulse width parameter PW via a signal or value 124 provided to thetransmit circuit 130 in order to control the pulse width of a singlepulse or burst of pulses provided by the generator circuit 132. Where asingle pulses generated by the transmit circuit 130, the pulse widthparameter PW in one embodiment directly sets the width of thetransmitted pulse. In certain embodiments where the transmit circuit 130provides a series of pulses or a burst, such as a pseudorandom pulsestream, the pulse width parameter PW such a total energy of the burst.

The processor 120 enables the receive circuit at 213 (high-going edge ofRXEN waveform 322 in graph 320 of FIG. 3), and provides the transmitenable control signal or value 122 to the transmit circuit 130(high-going edge of TXEN waveform 312 in graph 310) in order to causethe transmit circuit 130 and the optical source 106 to generate a firstoptical signal at the first end or location 102 a of the optical cable102 at 214 in FIG. 2. The processor 120 may provide a single transmitenable control signal to the transmit circuit 130 in certainembodiments, with the leading edge determining the start time of thetransmitted pulse and the trailing edge of the control signaldetermining the trailing edge, and hence the pulse width of thetransmitted pulse.

The receive circuit 140 samples the receive signal 142 at the firstsample rate 1/TS1 over a first time range R1 at 215, and the processor120 disables the receive circuit at 216 (e.g., using the receive enablesignal 126). In the illustrated example, the first time range R1 extendsfrom the transmission of the input pulse by the transmit circuit 130through a time TEND corresponding to the length of time necessary for alight pulse to travel from the first end 102 a of the cable 102 to thesecond end 102 b, and to be reflected back to the first end 102 a. Inother embodiments, the time range R1 may extend beyond TEND, or may beterminated before TEND, for example, if only a certain distance range ofinterest is being analyzed for defect locations in the optical cable102.

As seen in FIG. 3, the ADC 146 in the example embodiment employsperiodic sampling with a relatively long sample period TS1, where FIG. 3shows the sample points 306 illustrated along the horizontaltime/distance axis in the graph 300. In the example of FIGS. 3 and 4,sixteen samples 306, 406 are shown for ease of illustration, but anysuitable number of samples can be employed in various implementations.The receive circuit 140 thus obtains a first set of sample datacorresponding to reflections of the first optical signal. The processor120 temporally correlates the first set of sample data with thetransmitted first optical signal at 217 in FIG. 2, and identifies one ormore suspected defect locations at 218 based at least partially on thecorrelation. In the example of FIGS. 3 and 4, the processor 120identifies a first time TD1 (shown in graph 300 of FIG. 3) whichcorresponds to a suspected defect location DL1 (FIG. 1) based at leastpartially on a receive loss curve produced by the temporal correlation.Any suitable temporal correlation can be employed at 217, for example, astepwise multiplication of an input waveform corresponding to thetransmitted optical signal with the received data using accumulation,with the input waveform being stepwise shifted in time to deduce acorrelation curve representing time delay between the transmission ofthe input pulse and receipt of the receive pulse. The processor 120 inone example generates the curve 302 in FIG. 3 representing receive lossin dB having a discontinuity 304 at the time TD1, representing a drop inreceive signal strength a time TD1 after transmission of the input pulseinto first end 102 a of the optical cable 102. In this example, theprocessor 120 identifies one or more suspected defect locations byanalyzing the curve 302 with respect to discontinuities includingwithout limitation drops from a normal expected downward slope and/orspikes as seen at 304 in FIG. 3.

At 220 in FIG. 2, the processor 120 sets the ADC sample period of thereceive circuit 140 at 220 (e.g., via the Ts control signal or value 128in FIG. 1) to a shorter value TS2 (<TS1), and optionally sets thetransmit pulse width parameter PW of the transmit circuit 130 (e.g., viasignal or value 124) to a shorter value PW2 (<PW1) at 222. The processor120 implements a second (fine) defect location step at 230 in FIG. 2,including providing one or more further transmit control signals orvalues 122, 124 to cause the transmit circuit 130 and the optical source106 to generate and transmit a second optical signal at the first end102 a of the optical cable 102 at 231. In this example, the processor120 selectively enables the receive circuit at 232 by selectiveactuation of the enable signal or value 126 to sample the receive signal142 at the second sample rate (Fs=1/TS2) over the second time range R2following transmission of a second optical signal from the transmitcircuit 130 and the optical source 106. As seen in FIG. 4, moreover, thesecond time range R2 extends from T1 through T2 in the graph 400, andincludes the suspected defect location TD1, but his significantlyshorter than the original time range R1 used in the coarse processing ofFIG. 3. Although the second range R2 shown in FIG. 4 is generallyequally spaced on either side of the suspect location TD1, othersuitable second ranges can be implemented by the processor 120 in otherembodiments which include the suspect location TD1, but which need notbe symmetrically spaced around the suspect location TD1. As discussedfurther below in connection with FIGS. 7-9, however, the inventor hasappreciated that centering the fine processing range R2 around theinitially suspected location TD1 advantageously facilitates thepotential for subsequent discovery of multiple closely-spaced defectswithin the fine search range R2 using higher sample rates and preferablynarrower transmit pulse widths.

At 233 in FIG. 2, the samples are stored in the memory 147 of thereceive circuit 140 and/or in the memory 150, where the receive circuit140 may perform accumulation averaging in certain embodiments asdescribed above. In one possible implementation, the time range R2 isimplemented by the processor 120 selectively enabling the receivecircuit (curve 422 in graph 420 of FIG. 4) prior to TD1 parenthesese.g., at T1), and then disabling the receive circuit 140 at T2 after TD1(at 234 in FIG. 2), with the high-speed sampling at 1/TS2 providing asecond set of sample data corresponding to reflections of the secondtransmitted optical signal (samples shown at 406 in FIG. 4). Theprocessor 120 temporally correlates the second set of sample data withthe transmitted second optical signal at 235, for example, using theabove described correlation techniques, and identifies one or moredefect locations at 236 in FIG. 2 (e.g., defect location DL1 in FIG. 1)based at least in part on the temporal correlation. For example, theprocessor 120 can use discontinuity detection with respect to a receiveloss curve resulting from the temporal correlation in order to identifydefect locations at 236. Thereafter, the processor 120 may providedefect location information to a user, for example, by displaying acurve 162 on the display 160 as seen in FIG. 1, or by other graphicaland/or numeric indication.

As seen in the graph 400 of FIG. 4, the samples 406 in the finemeasurement are spaced much closer than the samples 306 in the coarseprocess shown in FIG. 3. In addition, the first and second sets ofsample data may include equal numbers of samples, although not a strictrequirement of all possible embodiments. This technique advantageouslyfacilitates relaxation of the memory requirements by selectively usinghigh sample rates in identified areas along the length of the opticalcable 102 at which a defect is suspected. In contrast, conventional OTDRtechniques utilizing high sample rates along the entire length L of theoptical fiber under test 102 would necessarily need to provide memorystorage for a significantly larger number of data samples. The process200, in contrast, selectively employs high sample rate data acquisitionat select locations (e.g., the second time range R2) within the overallrange R1 thereby employing the memory in an intelligent manner andallowing the system 102 to incorporate less memory 147, 150. Thisreduced or relaxed memory requirement, moreover, provides for powerefficiency of the OTDR IC 110, as the receive circuitry 140 need only beenabled during the second time range R2, as shown by the receive enablesignal curve 422 in graph 420 of FIG. 4, and since the overall powerdrawn by the system memory is reduced compared with conventional OTDRsystems having larger electronic memories.

Thus, the system 100 and the process 200 of the present disclosureadvantageously facilitate OTDR operations for locating defects in anoptical cable 102 for any cable length, and with any desired defectlocation resolution. It is noted that the initial or coarse processingat 210 can advantageously employ a relatively slow sample rate 1/TS1 anda relatively long transmit pulse width PW1 resulting in goodsignal-noise ratio (SNR), while providing a general indication ofsuspected defect locations at a coarse resolution, with the subsequentprocessing at 230 employing a higher sample rate, and potentially usinga narrower pulse width PW2 for improved resolution at the identifiedrange or ranges of interest. In certain embodiments, the processor 120sets the second sample rate 1/TS2 to be greater than or equal to aninteger K times the first sample rate 1/TS1, for example, where K>1,such as K=10 in one example. In this regard, the second sample rate1/TS2 can be set according to desired defect location identificationresolution specifications for a given application, and the resolution inthe initial coarse analysis at 210 is reduced by the factor K. Incertain embodiments, for example, the processor 124 sets the secondsample rate (1/TS2) to be greater than or equal to of the speed of lightin the optical cable under test 102 divided by half a desired or targetresolution value (twice the speed of light in the optical cable/thetarget defect location resolution), which may be provided by a user orwhich may be a predetermined value. Moreover, this technique can beemployed in certain embodiments to also reduce the memory requirementsby the factor K, for example, where the same number of samples isobtained in the coarse and fine sub-processes 210 and 230, respectively.

In addition, the processor 120 may intelligently set the transmit pulsewidth via the PW control signal or value 124 in the course and/or finesub-processes 210, 230 based at least partially on the selected samplerate. For example, the processor 120 in one embodiment sets the secondpulse width PW2 to be greater than or equal to twice the second sampleperiod TS2 (PW2≧(2/(1/TS2))), with the second pulse width PW2 being lessthan the first pulse width PW1 in certain implementations. The graph 410in FIG. 4 illustrates the use of a narrower pulse for the finemeasurement transmit enable signal waveform 412 having a width PW2 whichis less than the width PW1 in the enable signal waveform 312 used in thecoarse measurement of FIG. 3. As discussed further below in connectionwith FIGS. 7-9, this selective provision of a narrower pulse widthadvantageously facilitates differentiation between closely spaceddefects along the length of the cable under test 102.

FIGS. 5 and 6 provide graphs 500, 510, 520, 600, 610 and 620illustrating another example implementation for the situation in whichmultiple defects exist at locations DL1 and DL2 along the length of theoptical cable 102. In the coarse processing of FIG. 5, the samples 506are spaced by TS1 over the full range R1 implemented by the receiverenable waveform 522 (RXEN) in graph 520, and the transmitted pulse orburst is provided with a pulse width PW1 via the transmit enable signalwaveform 512 in graph 510 (TXEN). In this case, correlation of theinitial set of receive data samples identifies a first suspectedlocation at TD1 as well as a second suspected defect location at TD2based on discontinuities 504 and 508 in the coarse receive loss curve502 shown in FIG. 5. In this example, the processor 120 implements adual range fine analysis shown in FIG. 6, utilizing a narrower transmitpulse (pulse width PW2 implemented by the transmit enable signalwaveform 612 in graph 610), with the receive circuit 140 being enabledvia receive enable signal waveform 622 in graph 620 during a secondrange R2 from T1 through T2 (including TD1), as well as in a subsequentthird range R3 extending from T3 through T4 (including TD2). In anotherpossible embodiment, the processor 120 may instead utilize fine analysisover a single second range, for instance, from T1 through T4 using ahigher sample rate 1/TS2 to obtain a single second set of sample datafor correlation to verify/identify defect locations TD1 and TD2 based onthe initial suspected locations identified in the coarse processing.

Referring also to FIGS. 7-9, the system 100 and process 200advantageously provide for selective use of higher sampling rates toachieve a desired final resolution of the location of one or moredefects in the optical cable 102 under test. In certain implementations,the sample rate of the ADC 146 is set by the processor 120 in the fineprocessing at least partially according to a target defect locationresolution, e.g., 0.5 m for a 13 Km cable in one example, where thesampling period TS2 is set to be less than the time taken for light totravel the resolution distance. With respect to selective pulse widthadjustment by the processor 120, moreover, the received signal 142 fromthe optical sensor 108 is sampled, converted and correlated with thetransmitted signal to obtain the location of the defects. The graph 700in FIG. 7 shows an example transmitted optical signal curve 702 and areceived optical signal curve 704 in the case of a single detectabledefect at TD1 (e.g., again corresponding to the defect location DL1shown in FIG. 1 above). In this case, an example correlation curve 706is produced and analyzed by the processor 120 during the fine processingdescribed above to identify/confirm the suspected location TD1 asincluding a defect in the cable 102.

FIG. 8 shows a graph 800 illustrating a different situation in whichthere are two closely spaced defects at TD1 and TD2. As seen in FIG. 8,the use of a relatively wide pulse width PW1 in the transmitted opticalsignal curve 802 provides a received optical signal waveform 804presenting difficulties in identifying the separate defect locations TD1and TD2, where the correlation curve 806 does not include easilydistinguishable characteristics for these two defect locations. In thiscase, the processor 120 may identify only a single suspected defectlocation (e.g., midway between TD1 and TD2) during the initial or coarseprocessing.

As further seen in the graph 900 of FIG. 9, the selective employment ofa narrower pulse width PW2 in the transmitted optical signal curve 902during fine processing yields a received optical signal curve 904 havingseparately distinct pulse shapes. As previously mentioned, the lowerenergy in a narrow-width input pulse 902 sacrifices SNR, and thus thecorrelation signal waveform 906 in this case also has a lower amplitudecompared with that of FIG. 8, but results in distinct ramp shapesallowing the processor 120 to distinguish the two identifiable defectlocations at TD1 and TD2. In certain embodiments, the assessor 120advantageously provides the fine processing using a smaller pulse widthPW2 (<PW1), thus facilitating identification of multiple defectlocations even in situations where the initial coarse processingidentified only a single corresponding suspected defect location. Inthis regard, the processor 120 in certain embodiments maintains asampling period TS2 for the fine processing to be less than or equal tohalf the pulse width PW2 to ensure that the reflected pulsescorresponding to actual defect locations are separately identified bythe temporal correlation.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been disclosed with respect to only one of multipleimplementations, such feature may be combined with one or more otherfeatures of other embodiments as may be desired and advantageous for anygiven or particular application. Also, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in the detailed description and/or in the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

What is claimed is:
 1. A method for locating defects in an optical cable, the method comprising: transmitting a first optical signal at a first location of the optical cable; sampling optical reflections at the first location of the optical cable at a first sample rate over a first time range following transmission of the first optical signal to obtain a first set of sample data corresponding to reflections of the first optical signal; identifying a first time in the first time range corresponding to a suspected defect location of the optical cable based at least partially on a temporal correlation of the first set of sample data with the transmitted first optical signal; transmitting a second optical signal at the first location of the optical cable; selectively sampling optical reflections at the first location of the optical cable at a second sample rate over a second time range following transmission of the second optical signal to obtain a second set of sample data corresponding to reflections of the second optical signal, the second time range including the first time, the second time range being less than the first time range, and the second sample rate being greater than the first sample rate; and identifying at least one defect location of the optical cable based at least partially on a temporal correlation of the second set of sample data with the transmitted second optical signal.
 2. The method of claim 1, wherein the first optical signal is transmitted with a first pulse width, wherein the second optical signal is transmitted with a second pulse width, and wherein the first pulse width is greater than the second pulse width.
 3. The method of claim 1, wherein the second sample rate is greater than or equal to a constant K times the first sample rate, and wherein the constant K is greater than
 1. 4. The method of claim 1, wherein the second sample rate is greater than or equal to twice the speed of light in the optical cable/a target defect location resolution.
 5. The method of claim 4, wherein the first optical signal is transmitted with a first pulse width, wherein the second optical signal is transmitted with a second pulse width, and wherein the second pulse width is greater than or equal to 2 divided by the second sample rate.
 6. The method of claim 5, wherein the first pulse width is greater than the second pulse width.
 7. The method of claim 1, wherein the first optical signal is transmitted with a first pulse width, wherein the second optical signal is transmitted with a second pulse width, and wherein the second pulse width is greater than or equal to 2 divided by the second sample rate.
 8. The method of claim 7, wherein the first pulse width is greater than the second pulse width.
 9. The method of claim 1, comprising identifying multiple defect locations (DL1, DL2) of the optical cable based at least partially on the temporal correlation of the second set of sample data with the transmitted second optical signal.
 10. The method of claim 1, comprising: identifying a second time in the first time range corresponding to a second suspected defect location of the optical cable based at least partially on the temporal correlation of the first set of sample data with the transmitted first optical signal; selectively sampling optical reflections at the first location of the optical cable at the second sample rate in a third time range following transmission of the second optical signal to obtain a third set of sample data corresponding to reflections of the second optical signal, the third time range including the second time, and the third time range being less than the first time range; and identifying a second defect location of the optical cable based at least partially on a temporal correlation of the third set of sample data with the transmitted second optical signal.
 11. An optical time domain reflectometer (OTDR) system, comprising: a transmit circuit operative to generate a transmit signal defining at least one pulse; an optical source operative to transmit optical signals at a first location of an optical cable according to the transmitted signal from the transmit circuit; an optical sensor operative to receive optical reflections at the first location of the optical cable and to generate a receive signal according to the received optical reflections; a receive circuit operative to sample the receive signal at a selectable sample rate and to provide sample data corresponding to the optical reflections received by the optical sensor; an electronic memory operative to store the sample data; and at least one processor operatively coupled with the transmit circuit, the receive circuit, and the electronic memory, the at least one processor operative to: provide at least one transmit control signal or value to cause the transmit circuit and the optical source to generate a first optical signal at the first location of the optical cable, provide at least one receive control signal or value to cause the receive circuit to sample the receive signal at a first sample rate over a first time range following transmission of the first optical signal to obtain a first set of sample data corresponding to reflections of the first optical signal, temporally correlate the first set of sample data with the transmitted first optical signal, identify a first time in the first time range corresponding to a suspected defect location of the optical cable based at least partially on the temporal correlation of the first set of sample data with the transmitted first optical signal; provide at least one further transmit control signal or value to cause the transmit circuit and the optical source to generate a second optical signal at the first location of the optical cable, provide at least one further receive control signal or value to cause the receive circuit to sample the receive signal at a second sample rate over a second time range following transmission of the second optical signal to obtain a second set of sample data corresponding to reflections of the second optical signal, the second time range including the first time, the second time range being less than the first time range, and the second sample rate being greater than the first sample rate, temporally correlate the second set of sample data with the transmitted second optical signal, and identify at least one defect location of the optical cable based at least partially on the temporal correlation of the second set of sample data with the transmitted second optical signal.
 12. The OTDR system of claim 11, wherein the receive circuit comprises: a signal conditioning circuit operative to provide output signal based on the received signal from the optical sensor; an analog to digital converter circuit operative to sample the output signal of the signal conditioning circuit at the selectable sample rate and to provide the sample data; and a memory operative to store the sample data from the analog to digital converter.
 13. The OTDR system of claim 11, wherein the at least one processor is operative to provide the at least one receive control signal or value and the at least one further receive control signal or value to set the second sample rate of the receive circuit to be greater than the first sample rate.
 14. The OTDR system of claim 11, wherein the at least one processor is operative to provide the at least one further receive control signal or value to set the second sample rate of the receive circuit to be greater than or equal to twice the speed of light in the optical cable/a target defect location resolution.
 15. The OTDR system of claim 11, wherein the at least one processor is operative to: provide the at least one transmit control signal or value to cause the transmit circuit and the optical source to generate the first optical signal with a first pulse width; and provide the at least one further transmit control signal or value to cause the transmit circuit and the optical source to generate the second optical signal with a second pulse width, the second pulse width being greater than or equal to 2 divided by the second sample rate.
 16. The OTDR system of claim 15, wherein the at least one processor is operative to provide the at least one transmit control signal or value and the at least one further transmit control signal or value to set the first pulse width to be greater than the second pulse width.
 17. The OTDR system of claim 11, wherein the at least one processor is operative to provide the at least one transmit control signal or value and the at least one further transmit control signal or value to set the first pulse width to be greater than the second pulse width.
 18. An integrated circuit for an optical time domain reflectometer (OTDR) system, the integrated circuit comprising: a transmit circuit operative to generate a transmit signal defining at least one pulse for controlling an optical source to transmit optical signals; a receive circuit operative to sample receive signals from an optical sensor receiving optical reflections at a selectable sample rate and to provide sample data corresponding to the optical reflections; an electronic memory operative to store the sample data; and at least one processor operatively coupled with the transmit circuit, the receive circuit, and the electronic memory, the at least one processor operative to: provide at least one transmit control signal or value to cause the transmit circuit and the optical source to generate a first optical signal at a first location of an optical cable, provide at least one receive control signal or value to cause the receive circuit to sample the receive signal at a first sample rate over a first time range following transmission of the first optical signal to obtain a first set of sample data corresponding to reflections of the first optical signal, identify a first time in the first time range corresponding to a suspected defect location of the optical cable based at least partially on a temporal correlation of the first set of sample data with the transmitted first optical signal; provide at least one further transmit control signal or value to cause the transmit circuit and the optical source to generate a second optical signal at the first location of the optical cable, provide at least one further receive control signal or value to cause the receive circuit to sample the receive signal at a second sample rate over a second time range following transmission of the second optical signal to obtain a second set of sample data corresponding to reflections of the second optical signal, the second time range including the first time, the second time range being less than the first time range, and the second sample rate being greater than the first sample rate, and identify at least one defect location of the optical cable based at least partially on the temporal correlation of the second set of sample data with the transmitted second optical signal.
 19. The integrated circuit of claim 18, wherein the at least one processor is operative to provide the at least one transmit control signal or value and the at least one further transmit control signal or value to set the first pulse width to be greater than the second pulse width.
 20. The integrated circuit of claim 18, wherein the at least one processor is operative to provide the at least one further receive control signal or value to set the second sample rate of the receive circuit to be greater than or equal to twice the speed of light in the optical cable/a target defect location resolution. 